MOF-Triggered Synthesis of Subnanometer Ag02 Clusters and Fe3+ Single Atoms: Heterogenization Led to Efficient and Synergetic One-Pot Catalytic Reactions

The combination of well-defined Fe3+ isolated single-metal atoms and Ag2 subnanometer metal clusters within the channels of a metal–organic framework (MOF) is reported and characterized by single-crystal X-ray diffraction for the first time. The resulting hybrid material, with the formula [Ag02(Ag0)1.34FeIII0.66]@NaI2{NiII4[CuII2(Me3mpba)2]3}·63H2O (Fe3+Ag02@MOF), is capable of catalyzing the unprecedented direct conversion of styrene to phenylacetylene in one pot. In particular, Fe3+Ag02@MOF—which can easily be obtained in a gram scale—exhibits superior catalytic activity for the TEMPO-free oxidative cross-coupling of styrenes with phenyl sulfone to give vinyl sulfones in yields up to >99%, which are ultimately transformed, in situ, to the corresponding phenylacetylene product. The results presented here constitute a paradigmatic example of how the synthesis of different metal species in well-defined solid catalysts, combined with speciation of the true metal catalyst of an organic reaction in solution, allows the design of a new challenging reaction.


SI-2
: Well-formed dark green prisms of Fe 3+ Ag 0 2@MOF, which were suitable for X-ray diffraction, were obtained in a threestep PS process: First, crystals of Ni II 2{Ni II 4[Cu II 2(Me3mpba)2]3} · 54H2O (ca. 5 mg, 0.0015 mmol) were suspended, for 24 hours, in 5 mL of AgNO3 aqueous solutions (1 mg, 0.006 mmol), until complete replacement of Ni 2+ , cations hosted in the pores, by Ag + ones (assessed by SEM). Then, the resulting material was resuspended in a (NH4)2Fe(SO4)2 . 6H2O water/methanol (1:1) solution (1.2 mg, 0.003 mmol) under aerobic conditions. The process was repeated several times but the iron contents were identical to those obtained after 24 h. The crystals were isolated by filtration on paper and air-dried.
Alternatively, a multigram scale procedure was also carried out by using the same synthetic procedure but with greater amounts of both, a powder sample of compound

Physical Techniques
Elemental (C, H, N), and ICP-MS analyses were performed at the Microanalytical Service of the Universitat de València. FT-IR spectra were recorded on a Perkin-Elmer 882 spectrophotometer as KBr pellets. The thermogravimetric analysis was performed on crystalline samples under a dry N2 atmosphere with a Mettler Toledo TGA/STDA 851 e thermobalance operating at a heating rate of 10 ºC min -1 .
For each sample, five repeated measurements were collected at room temperature (2θ = 2-60°) and merged in a single diffractogram. A polycrystalline sample of Fe 3+ Ag 0 2@MOF was also measured after catalysis following the same procedure.
X-ray photoelectron spectroscopy (XPS) measurements: Samples of Fe 3+ Ag + @MOF and Fe 3+ Ag 0 2@MOF were prepared by sticking, without sieving, the MOF onto a molybdenum plate with scotch tape film, followed by air drying. Measurements were performed on a K-Alpha™ X-ray Photoelectron Spectrometer (XPS) System using a monochromatic Al K(alpha) source (1486.6 eV).
As an internal reference for the peak positions in the XPS spectra, the C1s peak has been set at 284.8 eV.
Thermogravimetric Analysis: The thermogravimetric analysis was performed on polycrystalline samples under a dry N2 atmosphere with a Mettler Toledo TGA/STDA 851 e thermobalance. The experiments were carried out within a temperature range from 25 °C up to 800 °C at a heating rate of 10 K/min. Approximately 20 mg of the membrane was placed in a ceramic pan for the measurements.
Microscopy measurements: Scanning Electron Microscopy (SEM) elemental analysis was carried out for Fe 3+ Ag 0 2@MOF, using a HITACHI S-4800 electron microscope coupled with an Energy Dispersive X-ray (EDX) detector. Data was analyzed with QUANTAX 400.
Gas adsorption: The N2 adsorption-desorption isotherms at 77 K, were carried out, on polycrystalline samples of Fe 3+ @MOF, Ag 0 2@MOF and Fe 3+ Ag 0 2@MOF with a BELSORP-mini-X instrument. Samples were first activated with methanol and then evacuated at 348 K during 19 hours under 10 -6 Torr prior to their analysis. Electronic paramagnetic resonance (EPR) measurements: The EPR measurements were performed at -170 ºC using an EMX-12 Bruker spectrometer working at the X band, with a frequency modulation of 100 kHz and 1 G amplitude. Portions at different times of each reactions were introduced inside an EPR quartz probe cell and were measured.

UV-Vis absorption and UV-
X-ray absorption spectroscopy (XAS) measurements were carried out on CLAESS beamline at ALBA Synchrotron Light Source, Barcelona (Spain). Together with the samples, several standard references (Fe foil, Fe2O3, Ag foil and Ag2O) have been finely powdered, uniformly mixed with cellulose, and pressed in pellets to ensure the correct absorption jump in fluorescence. Data reduction has been done using the Demeter program suit: raw data has been normalized by subtracting and dividing pre-edge and post-edge backgrounds as low order polynomial smooth curves. By assuming a linear dependency between the "white line" intensity (taken at the zero of the derivative spectra) and the corresponding electron valence (known for the set of reference compounds), we estimated the oxidation state of the sample. The local structure of the sample has been than refined using the EXAFS signal in the k range 3:12 Å -1 .
Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) measurements were performed in a double-aberration-corrected, monochromated, FEI Titan3 Themis 60-300 microscope working at 300 kV, by impregnating a gold filmed grid (Cu grids were not employed to measure the Cu content in the MOF) with a drop of SI-6 Fe 3+ Ag 0 2@MOF dispersed in dichloromethane and leaving evaporation for at least 5 hours. The microscope was also used to perform chemical mapping using the high-efficiency SuperX G2 detection system equipped in the microscope, which integrates four windowless detectors surrounding the sample and high-performance signal-processing hardware.
X-ray crystallographic data collection and structure refinement.
Crystal of Fe 3+ Ag 0 2@MOF with ca. 0.10 x 0.12 x 0.12 mm as dimensions was selected and mounted on a MITIGEN holder in Paratone oil and very quickly placed on a liquid nitrogen stream cooled at 150 K to avoid the possible degradation upon dehydration. Diffraction data were collected on a Bruker-Nonius X8APEXII CCD area detector diffractometer using graphite-monochromated Mo-Kα radiation ( = 0.71073 Å). The data were processed through SAINT 2 reduction and SADABS 3 multi-scan absorption software. The structure was solved with the SHELXS structure solution program, using the Patterson method. The model was refined with version 2018/3 of SHELXTL against F 2 on all data by full-matrix least squares. 4 Crystals of 3, suitable for X-ray diffraction, were obtained after the three-step PS process reported at page 3 of SI. For that reason, it is reasonable to observe a diffraction pattern sometimes affected by expected internal imperfections of the crystals. Furthermore, considering the huge cell in which compound crystallize and high porosity of the system, it is reasonable to observe a quite poor diffraction power of the samples even if in presence of heavy atoms as copper, nickel, iron and silver.
In fact, a completeness of data was obtained at θmax of 21°, (Table S1) (detected as Alerts A in the checkcifs). However, the solution and refinement parameters are suitable, compared with MOFs structures generally reported, thus we are convinced that the model structure found is consistent.
In the refinement non-hydrogen were refined anisotropically except disordered Fe 3+ , Na + and Ag + ions and lattice water molecules. All attempts to perform improved measurements on a single crystal of Fe 3+ Ag 0 2@MOF, resulting after a crystal-to-crystal transformation and featuring a very huge cell, either at I19 beamline of DIAMOND or in-house X-ray facilities failed, due to partial crystal damage / crystal deterioration under reduction conditions. The occupancy factors, of both Fe 3+ and Ag + ions have been defined in agreement with SEM results. The use of some C-C bond lengths restrains, SIMU SI-7 and DELU, SADI, DFIX and FLAT during the refinement has been reasonable imposed and related to flexibility of the three-substituted phenyl rings of the Me3mpba ligand that are dynamic components of the frameworks. In the refinement, some further restrains, to make the refinement more efficient, have been applied. For instance, ADP components have been restrained to be similar to other related atoms, EADP for group of atoms of the guest Fe 3+ and Ag + ions expected to have essentially similar ADPs.
The occupancy factors of Ag atoms and Fe 3+ ions have been defined in agreement with their thermal factors and SEM results [0.1667 for Fe1, 0.1075 for Ag1/Ag2 and 0.1250 for Ag3. Furthermore, it is important to underline that, depending to their occupancy, Fe 3+ metal ions are statistically disordered with Ag 0 2 dimers, exhibiting a random distribution with a population of 34 and 67% respectively.
The solvent molecules were disordered, only the molecules interaction with copper metal ions and in part with sodium ions have been in some way modelled. For that reason, in refinement, the contribution to the diffraction pattern from the disordered water molecules located in the voids was subtracted from the observed data through the SQUEEZE method, implemented in PLATON. 5 The hydrogen atoms of the ligand were set in calculated positions and refined as riding atoms whereas for detected water molecules were neither found nor calculated.
Overall the "Alert A" notifications found in the validation program CheckCIF are also related to intrinsic imperfections (as the presence of large outliers in the data set) quite normal for crystals that suffered a single-crystal to single-crystal process and disorder. They are unavoidable due to the expected severe disorder of both solvent and guest molecules. The comments for the main alerts A and B are described in the CIF using the validation reply form (vrf).
A summary of the crystallographic data and structure refinement for the Fe 3+ Ag 0 2@MOF compound is given in Table S1. Indeed, the somewhat high R values is, most likely, also affected by the contribution of the highly disordered solvent to the intensities of the low angle reflections. CCDC 2157534. 5 Spek, A. L. Acta Crystallogr. Sect. D, Biol. Crystallogr. 2009, 65, 148. SI-8 The final geometrical calculations on free voids (total potential solvent accessible void volume of 10156.2 Å 3 accounting for 52% of the cell volume) and the graphical manipulations were carried out with PLATON 7 implemented in WinGX, 6 and CRYSTAL MAKER 7 programs, respectively.

Catalysis details:
Reaction procedure for oxidative styrene couplings with soluble catalysts. Products 4 were prepared following the reaction scheme. Reagent 1 (1 eq, 0.4 mmol) and 2 (1 eq, 0.4 mmol) were introduced in a glass reactor equipped with a magnetic stirrer, together with K2S2O8 (2 eq, 0.8 mmol), TEMPO (0.2 eq, 0.08 mmol), AgNO3 (15% mol, 0.06 mmol) and 2 mL of toluene, and allowed to react overnight at 100 ºC under N2, after sealing the reactor. When reaction is complete, the resulting mixture is quenched by addition of water, extracted with dichloromethane and dried over Na2SO4. The products obtained are characterized by GC-MS. GC yields are obtained after using one equivalent respect to the limiting agent of an external standard (typically n-dodecane) and referring the obtained areas to the standard, following the formula: yield (product)= [(area product / response factor product) / (area standard / response factor standard)] x 100.
Reaction procedure for styrene couplings with solid MOF catalysts. Reagent 1 (1 eq, 0.05 mmol) and 2 (1 eq, 0.05 mmol) were introduced in a small glass vial with a magnetic stirrer, together with K2S2O8 (2 eq, 0.1 mmol), TEMPO (0.2 eq, 0.01 mmol), Fe 3+ Ag 0 2@MOF (11 mg, 5 mol% Ag) and 0.25 mL of toluene, and allowed to react for 24 h at 100 ºC under N2. When a combination of Fe 3+ @MOF (1.5 wt%) + Ag 0 2@MOF (4.4 wt%) was used, the amounts added of each catalyst were 11.5 mg (6.0 mol% Fe) and 11 mg (9.0 mol% Ag), to keep a total 15 mol% metal amount. After reaction is complete, the SI-9 mixture is filtrated to eliminate the catalyst, and the resulting liquid is extracted with dichloromethane and dried over Na2SO4. The products obtained are characterized by GC-MS.
Reaction procedure for vinyl sulfone conversion to phenylacetylenes with KO t Bu. Products 3 were prepared following the reaction scheme. Reagent 4 (1 eq, 0.12 mmol) was introduced in a glass reactor equipped with a magnetic stirrer and with a solution of KO t Bu (1M) in THF (2 eq, 0.24 mmol) at 70 ͦ C. When the reaction finishes, water is added, the mixture is extracted with THF and dried over Na2SO4. The products obtained are characterized by GC-MS.
Typical reaction procedure for catalyst reuse. Reuses of the Fe 3+ Ag 0 2@MOF solid catalyst were performed after separating the solids at the end of reaction by centrifugation, and washing the solid mixture with deionized water and methanol (three times) to remove excess reagent 2, TEMPO, K2S2O8, and any soluble product. Subsequently, the Fe 3+ Ag 0 2@MOF solid catalyst is dried under vacuum and directly use in the next reaction.
Hot-filtration test. Following the general reaction procedure above, the hot reaction mixture was filtered, at intermediate conversion, through a 0.25 μm Teflon filter. Filtrates were placed into a new glass reactor equipped with a magnetic stirrer and fresh insoluble reactant 2 and K2S2O8, and placed at the reaction temperature. The filtrates were periodically analyzed by GC, comparing the results obtained with the solid catalyst still in.
Reaction procedure for one-pot conversion of styrenes 1 to phenylacetylenes 3. Reagents 1 (1 eq, 0.05 mmol) and 2 (1 eq, 0.05 mmol) were introduced in a glass vial equipped with a magnetic stirrer, together with Fe 3+ Ag 0 2@MOF (11 mg, 5 mol% Ag), K2S2O8 (2 eq, 0.1 mmol) and 0.25 mL of toluene, and allowed to react for 24 h at 100 ºC under N2. After the reaction is complete, filtration is carried out SI-10 to remove the catalyst. The solution obtained is concentrated under vacuum, and then introduced in a glass reactor equipped with a magnetic stirrer, with the help of some THF solvent if necessary. A solution of KO t Bu (1 M) in THF (2 eq, 0.1 mmol) is then added, and the reaction stirred at 70 ºC for 1 h. When the reaction finishes, water is added, the mixture is extracted with THF and dried over Na2SO4.
Products 4 are characterized by GC-MS. Figure S1. View along c crystallographic axis of Fe 3+ Ag 0 2@MOF crystal structure showing randomly distribution of Fe 3+ (yellow spheres) and Ag 0 2 dimers (blue spheres) inside octagonal hydrophilic pores together with Ag 0 2 dimers formed and blocked in the small square pores of the porous network. Na + alkali metal ions are represented by orange spheres. Copper and nickel atoms from the network are represented by cyan and orange spheres respectively, whereas organic ligands are depicted as grey sticks. Figure S2. Perspective views of Fe 3+ Ag 0 2@MOF crystal structure emphasising Ag 0 2 dimers (blue spheres) located in large hydrophilic pores (right) and small square pores (left). Fe 3+ metal ions are depicted as yellow spheres. Copper and nickel atoms from the network are represented by cyan and orange spheres respectively, whereas organic ligands are depicted as grey sticks. Figure S3. Top (a) and side view (b) of a single channel displaying decorated pore walls by uniform distribution of active Fe 3+ isolated single metal atoms (yellow spheres) and Ag 0 2 subnanometric metal clusters (blue spheres). Copper and nickel atoms from the network are represented by cyan and orange spheres respectively, whereas organic ligands are depicted as grey sticks. Figure S4. Perspective view along c and b axis of Ag 0 2 dimers (blue spheres) residing in the small square pores of the porous network stabilized by Ag 0 ···Ooxamate interactions at 2.83(2) Å (blue dashed lines) and Na + alkali metal ions (orange spheres) connected to the walls of the network by means of non-covalent interactions. Copper and nickel atoms from the network are represented by cyan and orange spheres respectively, whereas organic ligands are depicted as grey sticks. Figure S5. Perspective view of Fe 3+ Ag 0 2@MOF crystal structures along b crystallographic axis clearly showing distribution of Ag 0 2 dimers (blue spheres) and Fe 3+ metal ions (yellow spheres) together with hydrated alkali Na + cations (orange spheres), retained in preferential cationic sites, further contributing to the robustness of the final material. Copper and nickel atoms from the network are represented by cyan and orange spheres respectively, whereas organic ligands are depicted as grey sticks. Figure S6. Theoretical (bold lines) and experimental (solid lines) PXRD pattern profiles of Fe 3+ @MOF (red), Ag 0 2@MOF (blue) and Fe 3+ Ag 0 2@MOF (green) in the 2θ range 2-60° (a) and 4-60° (b) for the sake of clarity. Figure S7. Backscattered SEM image of Fe 3+ Ag 0 2@MOF and the corresponding EDX elemental mapping for Cu (red), Ni (green) Fe (blue) and Ag (light blue) elements. The backscattering detector highlights the MOF particles as brighter areas due to crystalline MOF structure and to the presence of heavier atoms in the MOF than in the polymer matrix.             Tables. Table S1. Selected data from the ICP-MS a and SEM/EDX b analyses for Fe 3+ Ag 0 2@MOF.